Page 445 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Tudmapders 423

required to maintain thrust balance and prevent thrust bearing overload
and possible failure.
Special rotor blading arrangements permit unlimited condensation
through radial inflow expanders, or operation on flashing liquid streams
with no loss of efficiency or liquid erosion of the rotor blading.
Wheel resonance has been exhaustively studied. The results have been
computerized to provide wheels that are free of critical resonance condi-
tions within the normal operating range, and usually free of critical
speeds below trip speed. When a minor resonance occurs below design
speed, systems are available to bring the expander up to speed, automati-
cally passing rapidly up or down through the critical zone to prevent
wheel damage.
A solid state control system for the startup and speed control of multi-
ple , series-connected turboexpanders is available when required.

Perlodic Checks

Maintaining accurate running records for a machine is essential to ana-
lyze any problems that may occur. Operating data should be recorded at
least twice daily, and should include specific data on oil reservoir level,
thrust bearing pressures, oil temperatures and pressures. Running time
for the oil pumps (normally operated on a 30-day, equal-time basis)
should be included. An oil sample should be analyzed monthly to deter-
mine viscosity and change-out or purification schedules for the oil. Accu-
mulator charge pressure should be checked at least every two months to
ensure adequate protection for the bearings in the event of a shutdown.
(A typical periodic check sheet was reproduced earlier in Table 8-10.)

lVoubleshooting
Troubleshooting guidelines for expander operating problems follow:

Low Oil Pressure to the Bearings

This condition will actuate the alarm and start the auxiliary pump and,
if it reaches emergency proportions, will result in unit shutdown.
Check the lube system completely. Check filters for excessive pressure
drop, and verify the settings for all relief valves and pressure regulators.
Be sure to check the pressure-controlling bypass valve for excessive leak-
age. Sample the oil and test the viscosity. Reset the instruments and
gauges in the system as necessary. If the problem persists, the unit should
be disassembled and inspected for excessive bearing clearance.